This strays a little away from the intent of the original piece, but it still relates to arc length, so here goes:

I have a view of a cylindrical tank and have an accessory welded onto it at 18 degrees down from the horizontal center. I was hoping I could add an arc length dimension from center by selecting the horizontal center, the centerline of the accessory (which would give angle) and then the outside of the cylinder (which would give radius), but that doesn’t work.

Is there any way to show arc length dimensions of portions of a circle (as above) by using intersecting lines or similar? I don’t want to have to create an extra arc somehow just to get an arc length.

In case you’re wondering, the guys building the tank find it much easier to just grab a tape measure and measure down from center (or the edge of another feature), so it’s easiest way for me to communicate placement instead of using angles and radii.